The complement system consists of at least twenty chemically and immunologically distinct serum proteins comprising the complement components, the properdin system and the control proteins.Complement is a complex of nine different fractions,CI to C9. The fraction C1 occurs in serum as a calcium ion-dependent complex, which on chelation with EDTA yields three protein subunits called C1q, r, and

s. Thus C is made up of a total of 11 different proteins. C fractions are named C1 to C9 in the sequence of the cascading reaction, except that C4 comes after C1 before C2

The model traditionally used to explain C activity in immune cytolysis is the lysis of erythrocyte sensitised by its antibody. The erythrocyte (E) antibody (A) complex is called EA, and when C components are

attached to EA, the product is called EAC, followed by the components that have reacted (for example, EAC14235 or EAC 1-5). When a C component acquires enzymatic or other demonstrable biological activity, it is indicated by a bar over the component number, for example, the enzymatically activated C1 is shown as C1. Fragments cleaved from C components during the cascade are indicated by small letters (C3a, C3b). Inactivated forms of C components are indicated by

the prefix 'i' (iC3b).